1. Field of the Invention
The invention relates generally to the field of gas chromatography and specifically to the design of microwave heating apparatuses for heating a chromatographic column to achieve a controlled temperature profile along the length of the column.
2. Statement of the Problem
Gas chromatography is a physical method for the separation, identification, and quantification of chemical compounds. This method is used extensively for applications that include the measurement of product purity in analytical chemistry, the determination of environmental contamination, the characterization of natural substances, and the development of new products and processes.
A sample mixture to be analyzed in a gas chromatograph (GC) is injected into a flowing neutral carrier gas stream and the combination then flows through the chromatographic column. The inner surface of the column is coated with a material called the stationary phase. As the sample mixture and carrier stream flow through the column, the components within the mixture are retained by the stationary phase to a greater or lesser degree depending on the relative volatility of the individual components and on their respective affinities for the stationary phase. When the individual mixture components are released into the carrier stream by the stationary phase, they are swept towards the column outlet where they are detected and measured with a detector. Different chemical compounds are retained for different times by the stationary phase. By measuring the retention times, the specific compounds in the mixture can be identified. The relative concentration of the compounds is determined by comparing the peak amplitudes measured with the detector for each compound.
GC measurements are facilitated by the application of heat to the chromatographic column to change its temperature. The use of a heated column oven in gas chromatographic systems greatly increases the number of compounds that can be analyzed and speeds up the time required for each analysis by increasing the volatility of higher molecular weight compounds.
Many methods have been described for heating chromatographic columns. The simplest and most commonly used method utilizes resistive heating elements to heat air which is in turn circulated through an insulated oven in which the column is placed. For example, U.S. Pat. No. 3,527,567 to Philyaw et al. describes a GC oven heated with resistive elements.
The resistive element heating method has several limitations. To achieve even heating of the column, a large volume of air is rapidly circulated around the chromatographic column. In addition to heating the column, the air heats the oven itself. Because the thermal mass of the oven is much larger than that of the column, the rate at which the column can be heated is commensurately reduced. A related problem is cooling time. After heating the oven to a high temperature during an analysis, it takes significantly longer to cool the oven plus the column to their initial temperature so that the next sample may be analyzed than it would to cool the column alone. Together, these limitations reduce the throughput of the chromatograph.
Attempts to localize the resistive heat element onto the column itself so as to reduce or eliminate peripheral heating of the `oven` are described in U.S. Pat. No. 3,169,389 (Green et al.), U.S. Pat. No. 3,232,093 (Burow et al.), U.S. Pat. No. 5,005,399 (Holtzclaw et al.), and U.S. Pat. No. 5,808,178 (Rounbehler et al.). Each of these patents describe methods for directly wrapping or cladding the chromatographic column with a resistive heating element. Methods are also described for positioning the resulting metal clad column adjacent to a cooling source to decrease cooling times. This method of heating can be difficult to implement in practice because of uneven heating of the column due to local hot or cold spots in the resistive heating element surrounding the column or in the environment around the heating element. Uneven heating of the column in turn compromises the quality of the analysis.
Alternative methods for heating chromatographic columns by means of microwave heating are described in U.S. Pat. No. 4,204,423 (Jordan) and U.S. Pat. No. 5,808,178 (Rounbehler et al.). Potential advantages of microwave heating are selectivity, efficiency and speed. Suitable objects placed in a microwave oven will be heated when the oven is operated, but the oven itself will not be heated. Microwave heating occurs in materials which absorb microwave energy and convert it into heat. Thus, chroimatographic columns or column assembles which contain appropriate microwave absorbing materials will be selectively heated in a microwave oven while leaving the oven itself cool. Selective microwave heating makes possible more efficient heating because most thermal energy is transferred directly to the object to be heated. As compared to existing GC ovens, faster heating and cooling is also possible with microwave heating because less material is heated.
In order to heat a material in a microwave oven, the material must absorb microwave energy at least in part. Standard GC capillary columns are made of fused silica and polyimide. Neither of these materials absorb microwave energy appreciably. Consequently, these columns cannot be heated in a microwave oven in the manner taught by Jordan. The Applicant's co-pending U.S. patent application Ser. No. 09/108,297, entitled "Chromatography Column For Microwave Heating", addresses this limitation. It describes the design of chromatographic columns incorporating microwave absorbing material facilitating microwave heating.
There is another fundamental limitation in the method taught by Jordan. He does not describe any specific oven design in which a chromatographic column can be heated in a useful manner. To function properly, a GC column must not only be heated, it must be heated precisely. The temperature profile along the length of the column must be controlled within tight tolerances. For most applications in existing chromatographic ovens, the column temperature is kept essentially constant except for the ends which are usually maintained at a higher temperature. For other applications, it is useful to have a different temperature profile along the length of a column, such as a linear profile or a periodically varying profile.
Common capillary gas chromatographic columns range in size from 0.1 to 0.53 mm in internal diameter and from 4 to 60 meters in length. It is clearly impractical to build a microwave oven which heats a 60 meter column stretched out lengthwise. To get such a long chromatographic column into a practically sized oven, the column must be put into a more compact form. Chromatographic columns are usually quite flexible, especially fused silica capillary columns. Consequently, these columns can readily be coiled up into compact circular bundles having diameters as small as several centimeters (though other bundle shapes with appropriate bend radii can also be made). In bundled form, a chromatographic column can be heated within a microwave oven of practical size so that a desired temperature profile is achieved along the length of the column.
To achieve a desired temperature profile along the length of a coiled GC column with microwave heating, the microwave heating apparatus must be specifically designed to expose the coiled column to a specific electromagnetic field gradient along the column length. This is only achievable with a properly designed microwave heating apparatus and not with a generic microwave oven. Jordan does not describe how to design a microwave heating apparatus in which the electromagnetic field distribution is controlled in such a manner that a GC column is heated to a desired temperature profile.
Most gas chromatographs heat chromatographic columns isothermally (except for the ends) at a given time; i.e. the temperature of the entire column is kept at the same value at a given time though the specific isothermal temperature may change over time. The term "profile" is used herein to refer to the temperature versus position along the length of the column at a fixed point in time, as opposed to the temperature of the column as a function of time.
To isothermally heat a chromatographic column having a fixed microwave loss factor along its length in a microwave oven requires that the column must be exposed to the same electromagnetic field strength over its entire length (except for the ends which can be heated by the injector and detector assemblies) such that the whole of the column absorbs equal thermal energy and thus remains at the same temperature. Obtaining the desired temperature profile requires precise control of the electromagnetic field strength within the microwave oven. This cannot be achieved with conventional box-like, rectilinear, multi-mode microwave ovens where the electromagnetic field varies in an extremely complex manner throughout the oven volume. It can only be achieved with a microwave oven specifically designed to heat a chromatographic column.
3. Solution to the Problem
There are three essential features of a useful microwave oven to be used for heating coiled chromatographic columns: (1) The oven must be a single mode oven within which the electromagnetic field is predictable; (2) The electromagnetic field within the oven must have smoothly varying, continuous isofield lines oriented about a central axis such that individual coils of the column bundle can physically trace a path along or around the isofield lines resulting in a desired temperature gradient in each column coil; and (3) The electromagnetic field strength must vary from one coil to the next in the bundle by an increment sufficient to achieve the desired temperature gradient from one coil to the next.
The simplest column bundle geometry is that of a circular helix wherein each column coil is lined up adjacent to the next such that the overall bundle shape is that of a thin walled, constant diameter cylinder. To heat such a column bundle isothermally assuming the microwave absorption characteristics of the bundle do not significantly vary along the length of the column, the electromagnetic field strength must be equal (or nearly so) at all points in the cylindrical surface formed by the coiled column. Therefore, the electromagnetic field must be radially symmetric (rotationally invariant) and axially invariant. A microwave oven capable of generating such a field must itself be radially symmetric, i.e., circular in cross-section. Axially, it must be constructed to establish an equal electromagnetic field strength along in the length of the column bundle.
The present invention describes a number of chromatographic column microwave oven embodiments which can be used for heating chromatographic columns. Most such embodiments are radially symmetric in construction. Embodiments include coaxial transmission line ovens, coaxial resonant cavity ovens, and circular cylindrical resonant cavity ovens. A preferred embodiment of the oven is single mode resonant cavity and specifically a TM.sub.010 cylindrical resonant cavity. This cavity can be tuned to deliver virtually all available microwave power to the column heating element by adjusting the length of the coupling antenna.
If constructed in an axially invariant manner, all of these oven structures will typically have an electromagnetic field that varies in the axial direction. The TM.sub.010 cylindrical resonant cavity oven has the least variation. Several methods are described to alter the axial field distribution to which a chromatographic column bundle is exposed in a microwave oven so that a desired chromatographic column temperature profile is obtained. These methods include: (1) axially varying the geometry of the metal wall of the microwave oven; (2) axially varying the geometry of the inner conductor or central antenna; (3) axially varying the thickness of a dielectric insert in the oven; (4) axially varying the geometry of the coiled column or column heating element in the oven; or (5) axially varying the electromagnetic loss factor of the column heating element. In addition to these methods, a thermally conductive material can be used in the chromatographic column assembly to help eliminate any unwanted temperature variability that may exist.
There are two additional problems to be addressed in a useful chromatographic column microwave oven. The first concerns unwanted heat transfer from the column by air in the oven. The second concerns prevention of cold spots in the column ends where they are taken from the oven and connected to the injector and detector assemblies of the gas chromatograph. Methods are described herein for solving both problems such that the temperature profile of a column heated in a microwave oven is similar to that in conventional resistively-heated chromatographic ovens. In addition, methods are described for establishing useful column temperature profiles other than the conventional isothermal profile.